In semiconductor wafer fabrication processes, it is common to incorporate vacuum pumps to exhaust different types and concentrations of gases from process chambers. Cryogenic vacuum pumps (cryopumps) are often employed to evacuate gases from process chambers because they generally permit higher pumping speeds than other vacuum pumps. Cryopumps store most gases as solids condensed on the cryogenic surfaces of the pump or through cryogenic adsorption. High-boiling-point gases such as water vapor are condensed on the frontal array, while low-boiling-point gases, namely hydrogen, helium and neon, pass through the radiation shield and adsorb on the cryogenic surfaces of the pump. These surfaces may be coated with an adsorbent such as charcoal or a molecular sieve to adsorb the low-boiling-point gases.
After several days or weeks of use, the gases that are condensed onto the cryopanels, and in particular the gases that are adsorbed, accumulate and begin to saturate the cryopump. As the hydrogen accumulates on the pumping surfaces, the ultimate pressure for cryosorption pumping increases with time. This decreases the pumping capacity and speed of the pump. A regeneration procedure usually follows in order to warm the cryopump and release and remove the gases from the system. The pump, however, should only undergo regeneration when necessary because the typical regeneration process takes time during which the manufacturing or other process for which the cryopump creates a vacuum must idle. Therefore, it is desirable to determine exactly when the pump needs to be regenerated, and this can be facilitated by monitoring or predicting the absorption capacity of the pump. This factor is dependent upon the amount of adsorbent in the pump and is important because it determines the duration of running time between regenerations.
A mass spectrometer or quadrupole residual gas analyzer (RGA) can be used to monitor the adsorption capacity. These instruments, however, can be difficult to use because, among other things, the interpretation of the data output is often complex and ambiguous. Usually, the user needs to be familiar with the pattern in the spectrum to recognize the mass peaks detected to determine the pressure exerted by one gas in a mixture of gases. They are also relatively expensive devices.
The currently available mass spectrometer and RGA instruments do not fully achieve a cost effective, user-friendly, quick, simple and efficient solution for obtaining information about the adsorption capacity of low-boiling-point gases in a cryopump.